[Lldb-commits] [PATCH] Profile Assembly Until Ret Instruction
Tong Shen
endlessroad at google.com
Tue Aug 19 17:01:40 PDT 2014
Thanks Jason!
I will finish this patch and let's see how it goes.
P.S. I know a little about eh_frame stuff; I added CFI to the new Android
ahead-of-time Java compiler so AOT'ed code can properly unwind :-)
On Tue, Aug 19, 2014 at 4:51 PM, Jason Molenda <jmolenda at apple.com> wrote:
> The CIE sets the initial unwind state -- the CIE may describe the unwind
> state at the first instruction (as it always does with gcc, clang) but in
> theory it could describe the unwind state once the prologue had executed.
>
> The idea is that there is one CIE entry which describes a typical
> at-first-instruction unwind state and then many FDEs that describe the
> unwind instructions for specific functions - they all use that one CIE.
>
> Anyway, that's just an implementation detail of eh_frame. I honestly
> don't think we should worry about incomplete eh_frame - let's try living on
> them and see how it works in practice.
>
> It may be possible to categorize eh_frame to see how complete it is.
> Compiler-generated x86 prologues are very regular, it would be possible to
> look at the first few bytes of a function for some pushes or stack pointer
> changes and see if the eh_frame describes that. We know what the unwind
> state is on the first instruction of a function (it's determined by the
> ABI) -- does the eh_frame have the same instructions? Can we can through
> the function for an epilogue, and if we find one, does the eh_frame have
> unwind instructions there?
>
> But I don't want to have the perfect be the enemy of the good. IMO let's
> take the plunge and try, to use eh_frame and see how that goes. We can
> refine it later, or back it out again (it will be a very small change to
> RegisterContextLLDB) if necessary.
>
>
> > On Aug 19, 2014, at 4:41 PM, Tong Shen <endlessroad at google.com> wrote:
> >
> > And for no prologue case:
> > We can detect this easily (any CFI for start address?) and bail out, so
> we will fallback to assembly profiler.
> >
> >
> > On Tue, Aug 19, 2014 at 4:36 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > Ahh sorry I've been working on something else this week and didn't get
> back to you in time.
> > And you've been very patient and informative. Thanks!
> >
> > I'm only suggesting it for x86 / x86_64. What I am doing here relies on:
> > - Compiler describes prologue;
> > - We can figure our all mid function CFA changes by inspecting
> instructions.
> >
> > For frame 0, the new progress for CFA locating will look like this:
> > - Find the nearest CFI available before current PC.
> > - If the CFI is for current PC, viola :-) If not, continue.
> > - Inspect all instructions in between, and make changes to CFA
> accordingly. This can solve the PC relative addressing case.
> > - For epilogue, detect if we are in middle of an epilogue. Considering
> that there are not many patterns and they are all simple, I think we can
> enumerate them and handle accordingly.
> >
> > From what I've seen so far, this actually can solve most of gcc/clang
> generated code.
> > For JIT'ed code or hand written assembly, if there's no asynchronous CFI
> we are screwed anyway, so trying this won't hurt either (except some extra
> running time).\
> >
> > I hope I explain my thoughts clearly.
> >
> > Thank you.
> >
> >
> >
> > On Tue, Aug 19, 2014 at 4:22 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > Hi Tong, my message was a little rambling. Let's be specific.
> >
> > We are changing lldb to trust eh_frame instructions on the
> currently-executing aka 0th frame.
> >
> > In practice, gcc and clang eh_frame both describe the prologue, so this
> is OK.
> >
> > Old gcc and clang eh_frame do not describe the epilogue. So we need to
> add a pass for i386/x86_64 (at least) to augment the eh_frame-sourced
> unwind instructions. I don't know if it would be best to augment eh_frame
> UnwindPlans when we create them in DWARFCallFrameInfo or if it would be
> better to do it lazily when we are actually using the unwind instructions
> in RegisterContextLLDB (probably RegisterContextLLDB like you were doing).
> We should only do it once for a given function, of course.
> >
> > I think it would cleanest if the augmentation function lived in the
> UnwindAssembly class. But I haven't looked how easy it is to get an
> UnwindAssembly object where we need it.
> >
> >
> > Thanks for taking this on. It will be interesting to try living
> entirely off eh_frame and see how that works for all the
> architectures/environments lldb supports.
> >
> > I worry a little that we're depending on the generous eh_frame from
> clang/gcc and if we try to run on icc (Intel's compiler) or something like
> that, we may have no prologue instructions and stepping will work very
> poorly. But we'll cross that bridge when we get to it.
> >
> >
> >
> > > On Aug 15, 2014, at 8:07 PM, Jason Molenda <jmolenda at apple.com> wrote:
> > >
> > > Hi Tong, sorry for the delay in replying.
> > >
> > > I have a couple thoughts about the patch. First, the change in
> RegisterContextLLDB::GetFullUnwindPlanForFrame() forces the use of eh_frame
> unwind instructions ("UnwindPlanAtCallSite" - which normally means the
> eh_frame unwind instructions) for the currently-executing aka zeroth
> frame. We've talked about this before, but it's worth noting that this
> patch includes that change.
> > >
> > > There's still the problem of detecting how *asynchronous* those
> eh_frame unwind instructions are. For instance, what do you get for an
> i386 program that does
> > >
> > > #include <stdio.h>
> > > int main()
> > > {
> > > puts ("HI");
> > > }
> > >
> > > Most codegen will use a sequence like
> > >
> > > call LNextInstruction
> > > .LNextInstruction
> > > pop ebx
> > >
> > > this call & pop sequence is establishing the "pic base", it the
> program will then use that address to find the "HI" constant data. If you
> compile this -fomit-frame-pointer, so we have to use the stack pointer to
> find the CFA, do the eh_frame instructions describe this?
> > >
> > > It's a bit of an extreme example but it's one of those tricky cases
> where asynchronous ("accurate at every instruction") unwind instructions
> and synchronous ("accurate at places where we can throw an exception, or a
> callee can throw an exception") unwind instructions are different.
> > >
> > >
> > > I would use behaves_like_zeroth_frame instead of if (IsFrameZero())
> because you can have a frame in the middle of the stack which was the
> zeroth frame when an asynchronous signal came in -- in which case, the
> "callee" stack frame will be sigtramp.
> > >
> > >
> > > You'd want to update the UnwindLogMsgVerbose() text, of course.
> > >
> > >
> > > What your DWARFCallFrameInfo::PatchUnwindPlanForX86() function is
> doing is assuming that the unwind plan fails to include an epilogue
> description, steps through all the instructions in the function looking for
> the epilogue.
> > >
> > > DWARFCallFrameInfo doesn't seem like the right place for this.
> There's an assumption that the instructions came from eh_frame and that
> they are incomplete. It seems like it would more naturally live in the
> UnwindAssembly plugin and it would have a name like
> AugmentIncompleteUnwindPlanWithEpilogue or something like that.
> > >
> > > What if the CFI already does describe the epilogue? I imagine we'll
> just end up with a doubling of UnwindPlan Rows that describe the epilogue
> instructions.
> > >
> > > What if we have a mid-function epilogue? I've never seen gcc/clang
> generate these for x86, but it's possible. It's a common code sequence on
> arm/arm64. You can see a messy bit of code in
> UnwindAssemblyInstEmulation::GetNonCallSiteUnwindPlanFromAssembly which
> handles these -- saving the UnwindPlan's unwind instructions when we see
> the beginning of an epilogue, and once the epilogue is complete, restoring
> the unwind instructions.
> > >
> > >
> > > I'm not opposed to the patch - but it does make the assumption that
> we're going to use eh_frame for the currently executing function and that
> the eh_frame instructions do not include a description of the epilogue.
> (and that there is only one epilogue in the function). Mostly I want to
> call all of those aspects out so we're clear what we're talking about
> here. Let's clean it up a bit, put it in and see how it goes.
> > >
> > > J
> > >
> > >
> > >> On Aug 14, 2014, at 6:31 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > >>
> > >> Hi Jason,
> > >>
> > >> Turns out we still need CFI for frame 0 in certain situations...
> > >>
> > >> A possible approach is to disassemble machine code, and manually
> adjust CFI for frame 0. For example, if we see "pop ebp; => ret", we set
> cfa to [esp]; if we see "call next-insn; => pop %ebp", we set cfa_offset+=4.
> > >>
> > >> Patch attached, now it just implements adjustment for "pop ebp; ret".
> > >>
> > >> If you think this approach is OK, I will go ahead and add other
> tricks(i386 pc relative addressing, more styles of epilogue, etc).
> > >>
> > >> Thank you for your time!
> > >>
> > >>
> > >> On Thu, Jul 31, 2014 at 12:50 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > >> I think gdb's rationale for using CFI for leaf function is:
> > >> - gcc always generate CFI for progolue, so at function entry, we know
> the correct CFA;
> > >> - any stack pointer altering operation after that(mid-function &
> epilogue), we can recognize and handle them.
> > >> So basically, it assumes 2, hacks its way through 3 & 4, and pretends
> we are at 5.
> > >> Number of hacks we need seems to be small in x86 world, so this
> tradition is still here.
> > >>
> > >> Here's what gdb does for epilogue: normally when you run 'n', it will
> run one instruction a time till the next line/different stack id. But when
> it sees "pop %rbp; ret", it won't step into these instructions. Instead it
> will execute past them directly.
> > >> I didn't experiment with x86 pc-relative addressing; but I guess it
> will also recognize and execute past this pattern directly.
> > >>
> > >> So for compiler generated functions, what we do now with assembly
> parser now can be done with CFI + those gdb hacks.
> > >> And for hand-written assembly, i think CFI is almost always precise
> at instruction level. In this case, utilizing CFI instead of assembly
> parser will be a big help.
> > >>
> > >> So maybe we can apply those hacks, and trust CFI only for x86 &
> x86_64 targets?
> > >>
> > >>
> > >> On Thu, Jul 31, 2014 at 12:02 AM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > >> I think we could think of five levels of eh_frame information:
> > >>
> > >>
> > >> 1 unwind instructions at exception throw locations & locations where
> a callee may throw an exception
> > >>
> > >> 2 unwind instructions that describe the prologue
> > >>
> > >> 3 unwind instructions that describe the epilogue at the end of the
> function
> > >>
> > >> 4 unwind instructions that describe mid-function epilogues (I see
> these on arm all the time, don't see them on x86 with compiler generated
> code - but we don't use eh_frame on arm at Apple, I'm just mentioning it
> for completeness)
> > >>
> > >> 5 unwind instructions that describe any changes mid-function needed
> to unwind at all instructions ("asynchronous unwind information")
> > >>
> > >>
> > >> The eh_frame section only guarantees #1. gcc and clang always do #1
> and #2. Modern gcc's do #3. I don't know if gcc would do #4 on arm but
> it's not important, I just mention it for completeness. And no one does #5
> (as far as I know), even in the DWARF debug_frame section.
> > >>
> > >> I think it maybe possible to detect if an eh_frame entry fulfills #3
> by looking if the CFA definition on the last row is the same as the initial
> CFA definition. But I'm not sure how a debugger could use heuristics to
> determine much else.
> > >>
> > >>
> > >> In fact, detecting #3 may be the easiest thing to detect. I'm not
> sure if the debugger could really detect #2 except maybe if the function
> had a standard prologue (push rbp, mov rsp rbp) and the eh_frame didn't
> describe the effects of these instructions, the debugger could know that
> the eh_frame does not describe the prologue.
> > >>
> > >>
> > >>
> > >>
> > >>> On Jul 30, 2014, at 6:58 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > >>>
> > >>> Ah I understand now.
> > >>>
> > >>> Now prologue seems always included in CFI fro gcc & clang; and newer
> gcc includes epilogue as well.
> > >>> Maybe we can detect and use them when they are available?
> > >>>
> > >>>
> > >>> On Wed, Jul 30, 2014 at 6:44 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > >>> Ah, it looks like gcc changed since I last looked at its eh_frame
> output.
> > >>>
> > >>> It's not a bug -- the eh_frame unwind instructions only need to be
> accurate at instructions where an exception can be thrown, or where a
> callee function can throw an exception. There's no requirement to include
> prologue or epilogue instructions in the eh_frame.
> > >>>
> > >>> And unfortunately from lldb's perspective, when we see eh_frame
> we'll never know how descriptive it is. If it's old-gcc or clang, it won't
> include epilogue instructions. If it's from another compiler, it may not
> include any prologue/epilogue instructions at all.
> > >>>
> > >>> Maybe we could look over the UnwindPlan rows and see if the CFA
> definition of the last row matches the initial row's CFA definition. That
> would show that the epilogue is described. Unless it is a tail-call (aka
> noreturn) function - in which case the stack is never restored.
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>> On Jul 30, 2014, at 6:32 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > >>>>
> > >>>> GCC seems to generate a row for epilogue.
> > >>>> Do you think this is a clang bug, or at least a discrepancy between
> clang & gcc?
> > >>>>
> > >>>> Source:
> > >>>> int f() {
> > >>>> puts("HI\n");
> > >>>> return 5;
> > >>>> }
> > >>>>
> > >>>> Compile option: only -g
> > >>>>
> > >>>> gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
> > >>>> clang version 3.5.0 (213114)
> > >>>>
> > >>>> Env: Ubuntu 14.04, x86_64
> > >>>>
> > >>>> drawfdump -F of clang binary:
> > >>>> < 2><0x00400530:0x00400559><f><fde offset 0x00000088 length:
> 0x0000001c><eh aug data len 0x0>
> > >>>> 0x00400530: <off cfa=08(r7) > <off r16=-8(cfa) >
> > >>>> 0x00400531: <off cfa=16(r7) > <off r6=-16(cfa) > <off
> r16=-8(cfa) >
> > >>>> 0x00400534: <off cfa=16(r6) > <off r6=-16(cfa) > <off
> r16=-8(cfa) >
> > >>>>
> > >>>> drawfdump -F of gcc binary:
> > >>>> < 1><0x0040052d:0x00400542><f><fde offset 0x00000070 length:
> 0x0000001c><eh aug data len 0x0>
> > >>>> 0x0040052d: <off cfa=08(r7) > <off r16=-8(cfa) >
> > >>>> 0x0040052e: <off cfa=16(r7) > <off r6=-16(cfa) > <off
> r16=-8(cfa) >
> > >>>> 0x00400531: <off cfa=16(r6) > <off r6=-16(cfa) > <off
> r16=-8(cfa) >
> > >>>> 0x00400541: <off cfa=08(r7) > <off r6=-16(cfa) > <off
> r16=-8(cfa) >
> > >>>>
> > >>>>
> > >>>> On Wed, Jul 30, 2014 at 5:43 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > >>>> I'm open to trying to trust eh_frame at frame 0 for x86_64. The
> lack of epilogue descriptions in eh_frame is the biggest problem here.
> > >>>>
> > >>>> When you "step" or "next" in the debugger, the debugger instruction
> steps across the source line until it gets to the next source line. Every
> time it stops after an instruction step, it confirms that it is (1) between
> the start and end pc values for the source line, and (2) that the "stack
> id" (start address of the function + CFA address) is the same. If it stops
> and the stack id has changed, for a "next" command, it will backtrace one
> stack frame to see if it stepped into a function. If so, it sets a
> breakpoint on the return address and continues.
> > >>>>
> > >>>> If you switch lldb to prefer eh_frame instructions for x86_64, e.g.
> > >>>>
> > >>>> Index: source/Plugins/Process/Utility/RegisterContextLLDB.cpp
> > >>>> ===================================================================
> > >>>> --- source/Plugins/Process/Utility/RegisterContextLLDB.cpp
> (revision 214344)
> > >>>> +++ source/Plugins/Process/Utility/RegisterContextLLDB.cpp
> (working copy)
> > >>>> @@ -791,6 +791,22 @@
> > >>>> }
> > >>>> }
> > >>>>
> > >>>> + // For x86_64 debugging, let's try using the eh_frame
> instructions even if this is the currently
> > >>>> + // executing function (frame zero).
> > >>>> + Target *target = exe_ctx.GetTargetPtr();
> > >>>> + if (target
> > >>>> + && (target->GetArchitecture().GetCore() ==
> ArchSpec::eCore_x86_64_x86_64h
> > >>>> + || target->GetArchitecture().GetCore() ==
> ArchSpec::eCore_x86_64_x86_64))
> > >>>> + {
> > >>>> + unwind_plan_sp =
> func_unwinders_sp->GetUnwindPlanAtCallSite (m_current_offset_backed_up_one);
> > >>>> + int valid_offset = -1;
> > >>>> + if (IsUnwindPlanValidForCurrentPC(unwind_plan_sp,
> valid_offset))
> > >>>> + {
> > >>>> + UnwindLogMsgVerbose ("frame uses %s for full
> UnwindPlan, preferred over assembly profiling on x86_64",
> unwind_plan_sp->GetSourceName().GetCString());
> > >>>> + return unwind_plan_sp;
> > >>>> + }
> > >>>> + }
> > >>>> +
> > >>>> // Typically the NonCallSite UnwindPlan is the unwind created
> by inspecting the assembly language instructions
> > >>>> if (behaves_like_zeroth_frame)
> > >>>> {
> > >>>>
> > >>>>
> > >>>> you'll find that you have to "next" twice to step out of a
> function. Why? With a simple function like:
> > >>>>
> > >>>> * thread #1: tid = 0xaf31e, 0x0000000100000eb9 a.out`foo + 25 at
> a.c:5, queue = 'com.apple.main-thread', stop reason = step over
> > >>>> #0: 0x0000000100000eb9 a.out`foo + 25 at a.c:5
> > >>>> 2 int foo ()
> > >>>> 3 {
> > >>>> 4 puts("HI");
> > >>>> -> 5 return 5;
> > >>>> 6 }
> > >>>> 7
> > >>>> 8 int bar ()
> > >>>> (lldb) disass
> > >>>> a.out`foo at a.c:3:
> > >>>> 0x100000ea0: pushq %rbp
> > >>>> 0x100000ea1: movq %rsp, %rbp
> > >>>> 0x100000ea4: subq $0x10, %rsp
> > >>>> 0x100000ea8: leaq 0x6b(%rip), %rdi ; "HI"
> > >>>> 0x100000eaf: callq 0x100000efa ; symbol stub for:
> puts
> > >>>> 0x100000eb4: movl $0x5, %ecx
> > >>>> -> 0x100000eb9: movl %eax, -0x4(%rbp)
> > >>>> 0x100000ebc: movl %ecx, %eax
> > >>>> 0x100000ebe: addq $0x10, %rsp
> > >>>> 0x100000ec2: popq %rbp
> > >>>> 0x100000ec3: retq
> > >>>>
> > >>>>
> > >>>> if you do "next" lldb will instruction step, comparing the stack ID
> at every stop, until it gets to 0x100000ec3 at which point the stack ID
> will change. The CFA address (which the eh_frame tells us is rbp+16) just
> changed to the caller's CFA address because we're about to return. The
> eh_frame instructions really need to tell us that the CFA is now rsp+8 at
> 0x100000ec3.
> > >>>>
> > >>>> The end result is that you need to "next" twice to step out of a
> function.
> > >>>>
> > >>>> AssemblyParse_x86 has a special bit where it looks or the 'ret'
> instruction sequence at the end of the function -
> > >>>>
> > >>>> // Now look at the byte at the end of the AddressRange for a
> limited attempt at describing the
> > >>>> // epilogue. We're looking for the sequence
> > >>>>
> > >>>> // [ 0x5d ] mov %rbp, %rsp
> > >>>> // [ 0xc3 ] ret
> > >>>> // [ 0xe8 xx xx xx xx ] call __stack_chk_fail (this is
> sometimes the final insn in the function)
> > >>>>
> > >>>> // We want to add a Row describing how to unwind when we're
> stopped on the 'ret' instruction where the
> > >>>> // CFA is no longer defined in terms of rbp, but is now defined
> in terms of rsp like on function entry.
> > >>>>
> > >>>>
> > >>>> and adds an extra row of unwind details for that instruction.
> > >>>>
> > >>>>
> > >>>> I mention x86_64 as being a possible good test case here because I
> worry about the i386 picbase sequence (call next-instruction; pop $ebx)
> which occurs a lot. But for x86_64, my main concern is the epilogues.
> > >>>>
> > >>>>
> > >>>>
> > >>>>> On Jul 30, 2014, at 2:52 PM, Tong Shen <endlessroad at google.com>
> wrote:
> > >>>>>
> > >>>>> Thanks Jason! That's a very informative post, clarify things a lot
> :-)
> > >>>>>
> > >>>>> Well I have to admit that my patch is specifically for certain
> kind of functions, and now I see that's not the general case.
> > >>>>>
> > >>>>> I did some experiment with gdb. gdb uses CFI for frame 0, either
> x86 or x86_64. It looks for FDE of frame 0, and do CFA calculations
> according to that.
> > >>>>>
> > >>>>> - For compiler generated functions: I think there are 2 usage
> scenarios for frame 0: breakpoint and signal.
> > >>>>> - Breakpoints are usually at source line boundary instead of
> instruction boundary, and generally we won't be caught at stack pointer
> changing locations, so CFI is still valid.
> > >>>>> - For signal, synchronous unwind table may not be sufficient
> here. But only stack changing instructions will cause incorrect CFA
> calculation, so it' not always the case.
> > >>>>> - For hand written assembly functions: from what I've seen, most
> of the time CFI is present and actually asynchronous.
> > >>>>> So it seems that in most cases, even with only synchronous unwind
> table, CFI is still correct.
> > >>>>>
> > >>>>> I believe we can trust eh_frame for frame 0 and use assembly
> profiling as fallback. If both failed, maybe code owner should use
> -fasynchronous-unwind-tables :-)
> > >>>>>
> > >>>>>
> > >>>>> On Tue, Jul 29, 2014 at 4:59 PM, Jason Molenda <jmolenda at apple.com>
> wrote:
> > >>>>> It was a tricky one and got lost in the shuffle of a busy week. I
> was always reluctant to try profiling all the instructions in a function.
> On x86, compiler generated code (gcc/clang anyway) is very simplistic about
> setting up the stack frame at the start and only having one epilogue - so
> anything fancier risked making mistakes and could possibly have a
> performance impact as we run functions through the disassembler.
> > >>>>>
> > >>>>> For hand-written assembly functions (which can be very creative
> with their prologue/epilogue and where it is placed), my position is that
> they should write eh_frame instructions in their assembly source to tell
> lldb where to find things. There is one or two libraries on Mac OS X where
> we break the "ignore eh_frame for the currently executing function" because
> there are many hand-written assembly functions in there and the eh_frame is
> going to beat our own analysis.
> > >>>>>
> > >>>>>
> > >>>>> After I wrote the x86 unwinder, Greg and Caroline implemented the
> arm unwinder where it emulates every instruction in the function looking
> for prologue/epilogue instructions. We haven't seen it having a
> particularly bad impact performance-wise (lldb only does this disassembly
> for functions that it finds on stacks during an execution run, and it saves
> the result so it won't re-compute it for a given function). The clang
> armv7 codegen often has mid-function epilogues (early returns) which
> definitely complicated things and made it necessary to step through the
> entire function bodies. There's a bunch of code I added to support these
> mid-function epilogues - I have to save the register save state when I see
> an instruction which looks like an epilogue, and when I see the final ret
> instruction (aka restoring the saved lr contents into pc), I re-install the
> register save state from before the epilogue started.
> > >>>>>
> > >>>>> These things always make me a little nervous because the
> instruction analyzer obviously is doing a static analysis so it knows
> nothing about flow control. Tong's patch stops when it sees the first CALL
> instruction - but that's not right, that's just solving the problem for his
> particular function which doesn't have any CALL instructions before his
> prologue. :) You could imagine a function which saves a couple of
> registers, calls another function, then saves a couple more because it
> needs more scratch registers.
> > >>>>>
> > >>>>> If we're going to change to profiling deep into the function --
> and I'm not opposed to doing that, it's been fine on arm -- we should just
> do the entire function I think.
> > >>>>>
> > >>>>>
> > >>>>> Another alternative would be to trust eh_frame on x86_64 at frame
> 0. This is one of those things where there's not a great solution. The
> unwind instructions in eh_frame are only guaranteed to be accurate for
> synchronous unwinds -- that is, they are only guaranteed to be accurate at
> places where an exception could be thrown - at call sites. So for
> instances, there's no reason why the compiler has to describe the function
> prologue instructions at all. There's no requirement that the eh_frame
> instructions describe the epilogue instructions. The information about
> spilled registers only needs to be emitted where we could throw an
> exception, or where a callee could throw an exception.
> > >>>>>
> > >>>>> clang/gcc both emit detailed instructions for the prologue setup.
> But for i386 codegen if the compiler needs to access some pc-relative data,
> it will do a "call next-instruction; pop %eax" to get the current pc
> value. (x86_64 has rip-relative addressing so this isn't needed) If
> you're debugging -fomit-frame-pointer code, that means your CFA is
> expressed in terms of the stack pointer and the stack pointer just changed
> mid-function --- and eh_frame instructions don't describe this.
> > >>>>>
> > >>>>> The end result: If you want accurate unwinds 100% of the time, you
> can't rely on the unwind instructions from eh_frame. But they'll get you
> accurate unwinds 99.9% of the time ... also, last I checked, neither clang
> nor gcc describe the epilogue instructions.
> > >>>>>
> > >>>>>
> > >>>>> In *theory* the unwind instructions from the DWARF debug_frame
> section should be asynchronous -- they should describe how to find the CFA
> address for every instruction in the function. Which makes sense - you
> want eh_frame to be compact because it's bundled into the executable, so it
> should only have the information necessary for exception handling and you
> can put the verbose stuff in debug_frame DWARF for debuggers. But instead
> (again, last time I checked), the compilers put the exact same thing in
> debug_frame even if you use the -fasynchronous-unwind-tables (or whatever
> that switch was) option.
> > >>>>>
> > >>>>>
> > >>>>> So I don't know, maybe we should just start trusting eh_frame at
> frame 0 and write off those .1% cases where it isn't correct instead of
> trying to get too fancy with the assembly analysis code.
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>>> On Jul 29, 2014, at 4:17 PM, Todd Fiala <tfiala at google.com>
> wrote:
> > >>>>>>
> > >>>>>> Hey Jason,
> > >>>>>>
> > >>>>>> Do you have any feedback on this?
> > >>>>>>
> > >>>>>> Thanks!
> > >>>>>>
> > >>>>>> -Todd
> > >>>>>>
> > >>>>>>
> > >>>>>> On Fri, Jul 25, 2014 at 1:42 PM, Tong Shen <
> endlessroad at google.com> wrote:
> > >>>>>> Sorry, wrong version of patch...
> > >>>>>>
> > >>>>>>
> > >>>>>> On Fri, Jul 25, 2014 at 1:41 PM, Tong Shen <
> endlessroad at google.com> wrote:
> > >>>>>> Hi Molenda, lldb-commits,
> > >>>>>>
> > >>>>>> For now, x86 assembly profiler will stop after 10 "non-prologue"
> instructions. In practice it may not be sufficient. For example, we have a
> hand-written assembly function, which have hundreds of instruction before
> actual (stack-adjusting) prologue instructions.
> > >>>>>>
> > >>>>>> One way is to change the limit to 1000; but there will always be
> functions that break the limit :-) I believe the right thing to do here is
> parsing all instructions before "ret"/"call" as prologue instructions.
> > >>>>>>
> > >>>>>> Here's what I changed:
> > >>>>>> - For "push %rbx" and "mov %rbx, -8(%rbp)": only add first row
> for that register. They may appear multiple times in function body. But as
> long as one of them appears, first appearance should be in prologue(If it's
> not in prologue, this function will not use %rbx, so these 2 instructions
> should not appear at all).
> > >>>>>> - Also monitor "add %rsp 0x20".
> > >>>>>> - Remove non prologue instruction count.
> > >>>>>> - Add "call" instruction detection, and stop parsing after it.
> > >>>>>>
> > >>>>>> Thanks.
> > >>>>>>
> > >>>>>> --
> > >>>>>> Best Regards, Tong Shen
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> --
> > >>>>>> Best Regards, Tong Shen
> > >>>>>>
> > >>>>>> _______________________________________________
> > >>>>>> lldb-commits mailing list
> > >>>>>> lldb-commits at cs.uiuc.edu
> > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> --
> > >>>>>> Todd Fiala | Software Engineer | tfiala at google.com |
> 650-943-3180
> > >>>>>>
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>> --
> > >>>>> Best Regards, Tong Shen
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> --
> > >>>> Best Regards, Tong Shen
> > >>>
> > >>>
> > >>>
> > >>>
> > >>> --
> > >>> Best Regards, Tong Shen
> > >>
> > >>
> > >>
> > >>
> > >> --
> > >> Best Regards, Tong Shen
> > >>
> > >>
> > >>
> > >> --
> > >> Best Regards, Tong Shen
> > >> <adjust_cfi_for_frame_zero.patch>
> > >
> >
> >
> >
> >
> > --
> > Best Regards, Tong Shen
> >
> >
> >
> > --
> > Best Regards, Tong Shen
>
>
--
Best Regards, Tong Shen
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